Efficient Ultrathin Liquid Junction Photovoltaics Based on Transition Metal Dichalcogenides.

Ultrathin photovoltaics made of MoS2 or WSe2 have the potential to convert solar energy to electricity with high efficiency because all photogenerated carriers are produced at a charge-collecting interface. However, solid-state monolayer photovoltaic devices typically require that charge carriers travel parallel to, instead of perpendicular to, the three atom-thick material toward charge-collecting contacts. Parallel charge transport across long distances decreases energy conversion efficiency. Here we demonstrate proof-of-concept monolayer and bilayer TMD|I-,I3-|Pt photoelectrochemical solar cells that use a conformal liquid electrolyte junction for efficient perpendicular charge transport over significantly larger active areas than solid state systems. Efficient perpendicular charge transport is evidenced by high peak internal quantum efficiencies (IQE) of 44.2, 9.1, and 10.5% for 0.4 mm2 MoS2, WSe2, and MoS2/WSe2 domains in a predominantly monolayer MoS2/WSe2 film. The monochromatic energy conversion efficiencies are competitive with state-of-the-art solid-state monolayer heterojunction photovoltaics. However, inefficient light absorption limits the overall power conversion efficiency to 0.19% for this planar geometry monolayer photovoltaic system. Interestingly, correlated Raman and scanning photoelectrochemical microscopy measurements revealed a nonlinear scaling relation between IQE and layer thickness for MoS2, WSe2, and MoS2/MoS2/WSe2 domains within the heterojunction film. Specifically, the monochromatic energy conversion efficiency of bilayer MoS2 is an order of magnitude greater than monolayer MoS2 and MoS2/MoS2/WSe2 and it exceeds MoS2/WSe2 by a factor of 4. The structure/function relationships are hidden in ensemble-level photoelectrochemical measurements. Although nanostructured or plasmonic electrode architectures are still needed to enhance overall light absorption, our study shows that the liquid junction approach represents a simple and rapid strategy to screen ultrathin TMD materials combinations, tune interfacial energetics, and make conformal electrical contacts in photoelectrochemical energy conversion systems for electricity or solar fuels production.